72 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



explanation. I now pass to the resume of the alterna- 

 tive hypothesis, that the water ascends in the cavities 

 of the tracheides, vessels, &c., and not in the substance 

 of the cell-walls, 



In 1 88 1, Boehm published a paper on the subject, 1 

 which, whatever its shortcomings from the physicist's 

 point of view, must be quoted in order to show the 

 direction of thought on the side of those who could 

 not accept Sachs's assumptions, 



Boehm pointed out that the existence of the 

 " negative pressure" in transpiring plants puts osmosis 

 out of court as a cause for the ascent of the water in 

 the wood : he also agrees with those who reject all 

 capillary hypotheses. He then advances the following 

 criticisms ; (i) the wood contains more water than 

 can be contained in the walls ; (2) if cylinders of wood 

 are cut so that their long axes are parallel to a radius 

 of the stem, or to a tangent of the same, then the 

 easy pressure of water in the direction of their longi- 

 tudinal axes (which is known to occur in cylinders 

 with their long axes co-incident with the axis of the 

 stem) is no longer possible. In other words, it takes a 

 much greater pressure to drive water across the stem, 

 either tangentially or radially, than it does to drive 



r De fa cause du movement de feau, &c. (Ann. des Sc. Nat. vi. ser. 

 t. xii., 1881.) 



